In the days since Nigerian-born Umar Farouk Abdulmutallab (23) attempted to detonate explosives aboard Northwest flight 253 over Detroit, Obama has pledged to boost the number of armed marshals on US flights.

But there is scant evidence that the program has been able to deter or stop terrorist attacks.

The Federal Air Marshal Service shot to prominence in the wake of the September 11, 2001 attacks, when 200 000 people applied to join, but the program has been around since the late 1960s.

It began under former president John F. Kennedy and grew in strength as hijackings became increasingly frequent.

As airport security measures improved and metal detectors were installed, the program gradually dwindled.

"You teach them when and how to shoot"

By September 2001, "there were fewer than 50 federal air marshals... that were actively being deployed to detect, deter and defeat terrorism on board aircraft," said Nelson Minerly, a Transportation Security Administration spokesman.

The attacks, which killed nearly 3000 people, led to a massive growth in the program, though the exact number of marshals is classified.

The Transportation Security Administration (TSA) puts the figure at "several thousand," and various reports put the level at around 4000.

With a budget of $860-million, the program runs its own two-phase, 16 week training program for new recruits.

They are taught how to detect unusual behavior, detain a suspect and, all-importantly, fire a weapon aboard a flight in mid-air, said Mike Rush, the president of security company SCG International, which has trained air marshals.

"You teach them when and how to shoot and one of the most important things is not to penetrate the skin of the aircraft," he said.

"Shot placement, obviously, is very important and the type of ammunition you use. You use ammunition that does not penetrate through the body, that stops in the body."

"?The life of an air marshal is ultra-boring?"

Life as a marshal is hardly action-packed though.

Marshals "have to be patient, very patient. The life of an air marshal is ultra-boring 99 percent of the time," Rush said.

Air marshals have in fact rarely been called on to stop an attack, though they often deal with unruly or drunk passengers.

"I can't think of a single case where an air marshal has been reported to have been involved in stopping a terrorist incident," said Edward Alden, a border security expert at the Council for Foreign Relations.

The most useless agency?

Others have been more unkind.

In June 2009, Republican lawmaker John Duncan told a congressional hearing that the program was "probably the most needless, useless agency in the entire federal government."

"We now have approximately 4000 in the Federal Air Marshals Service, yet they have made an average of just 4.2 arrests a year since 2001."

Alden said boosting the air marshal program would be an inappropriate response to Abdulmutallab's failed attack.

"The air marshals are a last line of defense," he said. "I think the biggest thing that came out of this was the need to improve the watchlisting process."

Rush, a former Navy Seal, said the air marshal program can be extremely effective, but that Abdulmutallab should never have been able to board a plane.

"The problems with what happened on Christmas Day are not related to the air marshal program, that was pre-screening that didn't happen," he said.

Others believe the program has the potential to be effective, but only if it adds more marshals.

"An alert air marshal might have flagged Abdulmutallab," said James Jay Carafano, a security expert at the conservative Heritage Foundation.

"Currently, armed US air marshals cover only a fraction of international flights," he said in a policy brief.

"The US force should be expanded, and the White House should press allies to establish or expand their programs."

Alden noted that marshals may deter attackers or provide "psychological reassurance" to passengers, but he warned that intelligence coordination and screening would be more effective at stopping attacks.

The Obama administration is trying both, acknowledging that information given to US officials by Abdulmutallab's father was not properly processed.

Homeland Security Secretary Janet Napolitano said the administration would also "strengthen the presence and capacity" of air marshals, deploying law enforcement officers from the Homeland Security Department to fill that role.